Modulation system



Filed Aug. 6, 1925 5 m a m m m .m A

S a S.) m

Patented Jan. 21,1930;

,UNITED STATES PATENT OFFICE ROBERT L. DAVIS, 01 WILKINSBURGPENNSYLVANIA, ASSIGNOB TO WESTINGHOUSE I v ELECTRIC 8o MANUFACTURINGCOMPANY, A CORPORATION OFPENNSYLVANIA MODULATION SYSTEM Applicationfiled August 6, 1925 i Serial No. 48,433.

My invention relates to radio sending apparatus and particularly tomodulation systems therefor.

radio sending system adapted to be moduing system may be obtained bylated in accordance with audio-frequency vibrations and in whichimproved quality and more nearly complete modulation is obtained.

Another object of my invention is to pro- 1o vide a radio sendingapparatus with a balanced modulator system.

, Another object of my invention is to provide a radio sending system inwhich the oscillation generator and the modulator may be independentlysupplied with energy.

Modulation of a triode oscillation generatthe apparatus shown in Patent-#1,442,147 to Heising which discloses the so-called constant currentsystem. This system, however, is lim ited by the fact that it is notpossible to obtain complete modulation of the oscillation generatorwithout producing at the same time very serious distortion of the soundfrequency modulation. This is by reason of the fact that the resistanceof the modulation triode changes in strict linear proportionality to thevoltage impressed on the rid only over a relatively narrow range 01:change in grid voltage. For this reason, neither the maximum currentcarrying capacity of the modulator triode nor zero current capacity isavailable to change the current through the oscillation generator. Theconventional modulator triode system is thus seriously overloaded andcomplete modulation of the oscillation generator is not obtained,thereby sacrificing the distance range and the quality of the modulationof the system.

My inventionprovides means whereby two modulator triodes may be made tooperate with a single oscillation generating triode and whereby thedeviations of one tube from linear reproduction are compensated'by deviations in'the opposite direction in another tube.

Other objects and structural details of my invention will be apparentfrom the following description when read in connection with theaccompanying drawings, wherein Fig. 1 is a diagrammatic representationof form of my invention. An object of my invention is to prov1de a c inthe electrical supply sources from influencing the oscillator triode,and for preventmg the presence in the core of the modulationtransformer, oi a flux produced by the direct current source of platepotential.

Referring to the drawing, an oscillation radiating system "1 is providedhaving the customary aerial, counterpoise, and inductance elements. Theradiating system 1 is connected to a local oscillating circuit 2 whichis energized by the triode oscillator 3; Modulator triodes 4 and 5 areprovided, the grids of which are connected to opposite terminals of thesecondary coil 6 of the modulation transformer 7. The primary winding ofthe transformer 7 is connected to a microphone 8 and a battery 9.

The respective plate circuits of the triodes 4 and 5 are connected toopposite terminals of the primary coil 11 of an output transformer 12.The mid-point of the coil 11 is L connected to a source of electricalenergy.

The cathodes of the respective triodes are en-' ergized by a generator14.

In the embodiment of my invention shown in Fig. 1, a singlehigh voltagesource 15 is provided for energizing both the modulator triodes and theoscillator triode, connection being made from it through the secondarycoil 16 of the transformer 12 to the plate circuit of the triode 3 andin parallel therewith to the mid-point of the primary winding 11 of thetransformer 12 and through the wmding to the respective plates of thetriodes 4 and 5. A by-pass condenser 17 is provided to bypass theaudio-frequency component of the plate current to the triode 3, and isconnected in shunt to the generator 15, thereby allowing theaudio-frequency vibratory component to flow through a circuit other thanthe highly inductive windings of the generator 15 and also tending toremove any current fluctuations produced by the generator 15.

In the operation of this embodiment of my invention, current is suppliedfrom the generator 15 through the secondary coil 16 to the oscillatorgenerator 3. Current is simultaneously supplied through the primary coil11 to the late circuits of the triodes 4 and 5. The mlcrophone 8produces a vibratory current under the influence of sound, which currentcauses the develo ment of potentials in the current coils 6, wliichpotentials are of opposite polarity and are impressed upon therespective grids of the triodes 4 and 5. The otentials cause adiminution in current t rough one of the triodes 5 and 6, and

an increase in current in the other.

The resulting current changes in the halves of the coil 11 produced bythe change in impedance of the respective triodes 4 and 5 causes anintegrated change in the induction of the coil 11. This generates apotential in coil 16 which is twice the potential developed by the samecurrent change through a single one of the triodes. The current changesin the triodes 4 and 5 are integrated even though they are in oppositedirections- Thepotential generated in coil 16 changes the current flowthrough the triode 3 and thereby changes the amplitude of theoscillations generated by it in the well-known manner. The vibratorycomponent of the current flowing in this triode is by-passed through thecondenser 17. A substantially constant current flows to the tap of coil11 at all times and fluctuations in its value are without eflect uponthe coil 16 since the direction of current through the respective coilscauses the neutralization of the magnetizin effect of the steadycurrent.

The act that the grids of the two triodes 4 and 5 are changed inpotential in opposite directions results in a compensation of anydeviation from linear reproduction which may occur since one triode maybe approaching the toe of its characteristic curve while the othertriode is approaching the knee of its characteristic curve and alessened change of conductivity of one triode is compensated b theincreased change in conductivity of tlie other. Likewise, the output ofboth triodes is available for modulating the ouput of the oscillator 3because a current chan e of only half the total capacity of theindividual triodes 4 and 5 will result in the same degree of modulationof the oscillator 3 as is produced by system of the Heising Patent.

The halves of the coil 11 may be of the same size as coil 16, in whichcase, the above statement is correct, or the halves of coil 11 may belarger than coil 16, in which case a greater amount of modulation may beobtained.

The embodiment of Fig. 2 contains one generator 18 to supply the platecurrent of the oscillator 3 and a second generator 19 to supply themodulator triodes 4 and 5 by the system of connections shown. By thismeans, it is possible to employ triodes having different characteristicsfor modulators from those employed as oscillators and to supply themwith a suitable potential. An additional filter inductance 21 isprovided in the circuit of the generator 18 and the triode 3 as a meansfor steadyin'g the generator output. A condenser 22, correspondin denser17 of Fig. 1, is inserted between the filter inductance coil 21 and thecathode circuit. An ammeter 23 may be-inserted in series with thecondenser 22 and, when properly calibrated, will indicate the percentageof modulation of the generator.

The operaton of the embodiment of Fig. 2 is substantially similar tothat of Fig. 1 exto the con- I cept that two separate current sourcesare 7 provided.

The embodiment of Fig. 3 shows a construction adapted to embody a singleplate current generator for both modulator and oscillator with means forremoving any generator fiuctuations from the plate current of theoscillator triode 3. In this embodiment, a terminal of the secondarycoil 16 is connected through a condenser 24 directly to the cathodecircuit of the oscillator 3. The generator 25 is then connected betweenthe cathode circuit and the mid-tap of coil 11 directly, and thegenerator 25 is also connected through a filter choke inductance 26 tothe plate circuit of the oscillator 3.

In the operation of the embodiment shown in Fig. 3, the modulatorportion functionsin a manner substantially similar to that of Fig. 1,and the vibratory energy induced in the coil 16 is impressed upon'theplate circuit of the triode 3 through the condenser 24. The platecircuit is then supplied with a steady current through the filter chokeinductance 26. This construction prevents the presence in the core ofthe transformer 12 of any flux produced by the direct current source ofpotential for the plate of oscillator triode 3, as none of theoscillation direct current su ply passes through the coil 16 and thestea y currents in the one half of coil 11 neutralizes that of the otherhalf. 7

By the means of construction shown, I am enabled to provide in a radiosending system modulating means whereby complete modulation oftheoscillation generator is obtained without overloading the oscillatortriodes and whereby deviations of the characteristics of the modulatorsare compensated for, and whereby a plurality of plate energy sources maybe employed.

While I have shown but three embodiments of my invention, it is capableof other modifications without departing from the spirit thereof and itis, therefore, desired that only such limitations be imposed thereon asare imposed by the prior art or indicated in the appended claims.

I c aim as my invention:

1. In a radio system, an oscillator tube,

two modulator tubes, a source of late current, a transformer,connections .rom said source to the respective plates of said modulatortubes in parallel, said connections including a mid-tap to the primaryof said transformer, and connections for the plate of said oscillatortube includin said source and the secondary of said trans ormer inparallel, the branch of said last-named connection including saidsecondary also including means for preventing the passage of directcurrent.

2. In a radio system, an oscillator tube, two modulator tubes, 3. sourceof plate current for said modulator tubes, a transformer having aprimary winding provided with a center tap, a secondary winding, saidsecondary wlnding being included serially with a block ing condenserbetween the plate and the cathode of said oscillator tube, and aconnection between the positive terminal of said source and said centertap, whereby said transformer is prevented from saturating.

3. In a radio system, an oscillation generator, a plurality of modulatordevices, connec o tions whereby said devices influence said 0scillationgenerator in push-pull relation, said connections including athree-winding transformer, and means for preventing the flow of directcurrent in the secondary winding of as said transformer, wherebysaturation thereof is prevented.

In testimony whereof, I have hereunto subgcribed my name this 11th dayof July, 192

4o ROBERT L. DAVIS.

